A typical prior art configuration of an electromagnetically operated latch is seen in FIG. 1. Here, an electromagnet has a U-shaped yoke or core structure 1 supported upon a frame 2. The yoke mounts a coil 3 on one leg 1a. A latch lever 4, having a pole end 4a and a latch end 4b, mounts a keeper 4c on the pole end 4a for angular movement about the axis of a pivot 5 on the frame 2. The axis of the pivot is centered in a position above the end of the other leg 1b of the yoke 1, defining a magnetic gap 6. The lever 4 is biased by a spring 7 to a first position, the latch position, which is the position shown in FIG. 1. The keeper 4c on the lever 4 defines a variable length magnetic gap 8 with the pole tip 1al on the end of the yoke leg 1a on which the coil 3 is wound. When the coil 3 is energized, the force of flux coupling at the magnetic gap 8 rotates the lever 4 clockwise against the force of the spring 7. Flux coupling with the keeper at the magnetic gap 6, being centered on the axis of the pivot 5, produces no useful torque. In the latched position of the lever 4, the magnetic gap 8 is large and the gap reluctance is high, requiring a high coil current to move the lever 4 from latched position, which is undesirable. Keeping the gap 8 small, limits angular movement of the lever 4, necessitating a long length of the latch end 4b of the lever 4. Making the latch end 4b of the latch lever 4 longer than the pole end 4a can provide adequate displacement of the latch hook 4d with limited angular displacement of the latch lever 4 for latching and releasing purposes.
Having the magnetic gap 8 close to the pivot 5 of the lever 4, minimizes the dimension of the magnetic gap 8 in the latched position of the lever 4, but the electromagnetic moment arm 4a being shod requires higher flux density in the gap 8 to achieve the required torque for operating the lever. The primary disadvantage of this design is that it is difficult to scale the electromagnetic latch to a smaller device such as a smaller form factor disk drive. If the distance of travel of the latch hook 4d is fixed, an even greater difference between the pole end length and the latch hook end length is required if the overall size is to be reduced.
A design of an electromagnet is needed in which the variation of the reluctance of the magnetic circuit between latched and unlatched angular positions of the latching lever 4 is minimal to permit increased angular displacement of the latch lever 4 while minimizing operating current requirements.
It is also evident that an electromagnetic actuator latch design is needed which utilizes a minimum of electric power to move the latch lever to actuator released position and to maintain the latch lever 4 in that position, in continuous use of the disk drive.